Leave promised conflicting things to different groups, to get them all on board
e.g. look at the "Breaking Point" poster and scare videos, vs targeted ads at Asian minorities promising easier immigration for their relatives
So they had to pretend that after Brexit everyone would continue to enjoy the advantages of the EU, its Single Market and agencies, while not being bound by the rules of the EU like members are
This was a highly effective - although completely dishonest - way to snatch a narrow majority in a referencdum,
but then makes it impossible to achieve a Brexit that satisfies even most Leavers, let alone the country as a whole
e.g.
"easiest trade deal in history"
"we hold all the cards"
" only an idiot would leave the SIngle MArket"
"Why can't we be like Norway"
are promises of an entirely different Brexit to the bare bones post-Brexit deal that is now BJ's best hope